The so-called Claus process is commonly used to convert hydrogen sulfide to elemental sulfur.
A typical three converter-Claus plant, may report an efficiency in sulfur recovery of about 97 percent under optimal conditions. However, actual environmental legislation restricts SO.sub.2 emissions levels to about 250 ppm/vol or its equivalent, in other words, a global recovery of 99 to 99.7 percent. To achieve such low emission of SO.sub.2, an additional process is necessary for deep sulfur recovery to enable a more than 97 percent recovery using the Claus process, reducing in this way SO.sub.2 emissions within the accepted limits. Such additional process is applied in a stage after the Claus process, and mainly consists in a selective oxidation to transform the remaining gas to elemental sulfur and water.
U.S. Pat. No. 4,818,740 describes the Claus process and discloses selective oxidation of sulfur-containing compounds resulting from the Claus process using a Fe.sub.2 O.sub.3 --Cr.sub.2 O.sub.3 /.alpha.-Al.sub.2 O.sub.3 catalyst in concentrations of 0.5 and 4.5 weight percent, respectively, achieving a conversion of acid gas to sulfur, above 90 percent. The disclosure of U.S. Pat. No. 4,818,740 is incorporated by reference in its entirety.
U.S. Pat. No. 4,311,683 discloses H.sub.2 S conversion to sulfur, by the use of a vanadium oxide and vanadium sulfide catalyst supported on a porous oxide which is non-alkaline. The disclosure of U.S. Pat. No. 4,311,683 is incorporated by reference in its entirety.